The invention relates to a process for obtaining a photochromic compound and also to a photochromic compound obtained according to this process.
It applies more particularly to the manufacture of photochromic glazing employed either in building or in vehicles.
Processes of this type are described, for example, in documents DE 2 707 602 A1 and WO 91/16644. In these known processes a photochromic emulsion of silver halide is produced by reacting dissolved silver salts with silver halides, this being done in the presence of polymer binders. The polymer binders must be chosen so that they bind the halides reversibly. In addition, it is also known to dope silver halide crystals by means of copper ions, this being not only within the bulk but also on the surface. In addition, it is known that the photochromic effect of the emulsion can be improved by adding other metal salts, for example cobalt, chromium, manganese, magnesium or rare-earth salts.
Whereas in the case of the known photochromic glasses, in which the solver halides are dispersed in the glass matrix, each silver halide centre is visibly independent of the others insofar as its photochromic properties are concerned, it seems that this is not the case with polymer matrices. The known photochromic compounds based on a polymer matrix tend in any event no longer react in a completely reversible manner in the course of time and to form dispersion centres, with the result that the layers exhibit an increasing level of light scatter. To improve the behaviour of such compounds it is proposed in documents DE 2 707 602 A1 and WO 91/16644 to give the polymer matrix a determined minimum of electrical conductivity by adding to the emulsion, for example, acids, ammonium salts or glycerine.